Reflection loss suppression circuit

ABSTRACT

An object is to suppress a reflection loss in any desired frequency band outside a required band, without affecting circuit characteristics in the required frequency band. The output impedance of a circuit in a specified frequency band is transformed into a high impedance by a transmission line  15 . A resistance grounding circuit  18  having a frequency selectivity is connected in parallel. The resistance grounding circuit  18  is constructed of a resistor  16  which has a resistance close to a load resistance, a capacitance C which is selected so as to satisfy an equation Im[tan h{γ(λ/2−δ)}]=−ω 0 CZ 0  at an angular frequency ω 0  included in the frequency band, and a (λ/2−δ)-long end-open stub.

TECHNICAL FIELD

The present invention relates to a reflection loss suppression circuitfor use in various microwave/milliwave circuits and digital circuits.

BACKGROUND ART

Heretofore, microwave/milliwave circuits which perform theamplification, oscillation, mixing, etc. of radio frequency signals, anddigital circuits which perform the amplification, identification,branch, etc. of digital signals have been put into practical use invarious systems.

Among the various circuits, a wideband amplifier of distributed type foruse in an optical communication system or a radio communication systemwill be described with reference to FIG. 1. FIG. 1 is a circuit diagramshowing an example of the distributed amplifier in the prior art.

The distributed amplifier in the prior art is a cascode-type distributedamplifier of three-stage configuration as employs HBT cascode pairs 3each being constructed by cascode-connecting a hetero-junction bipolartransistor (hereinbelow, abbreviated to “HBT”) 1 and an HBT 2. An inputsignal is received from an input terminal 7, while an output signal isdelivered from an output terminal 8. The base terminal of the HBT 1 isfed with a DC supply voltage from a base power source 9 through aterminating resistor 12. The collector terminal of the HBT 2 is fed witha DC supply voltage from a collector power source 10 through aterminating resistor 12. The base terminal of the HBT 2 is fed with a DCsupply voltage from a cascode power source 11 through a terminatingresistor 12. Besides, the base terminal of the HBT 2 is groundedradio-frequency-wise through an RF grounding capacitor 13.

The distributed amplifier stated above forms a transmission line of highcutoff frequency by combining the parasitic reactances of the HBTs 1 andHBTs 2 with high-impedance transmission lines 4 and 5. The transmissionline has a characteristic impedance which is equal to each of a signalsource impedance and a load impedance, and it can realize a flat gainand a low reflection loss over a wide band.

With the cascode-type distributed amplifier in the prior art as shown inFIG. 1, however, a reflection loss particularly on an output sideincreases outside a required frequency band. There has accordingly beenthe problem that the reflection loss leads to the occurrence of anegative resistance in some cases. There has also been the problem thatthe stability of the circuit is consequently degraded to give rise to aparasitic oscillation or an unstable operation.

Incidentally, although the distributed amplifier has been exemplifiedhere, the problems explained above are problems which can occur invarious circuits such as microwave/milliwave circuits which perform theamplification, oscillation, mixing, etc. of radio frequency signals, anddigital circuits which perform the amplification, identification,branch, etc. of digital signals.

The present invention has been made in view of the above problems. Anobject of the present invention is to connect a reflection losssuppression circuit to any of various circuits including a distributedamplifier, whereby a reflection loss outside a required frequency bandis satisfactorily suppressed without degrading the characteristics ofthe circuit in the required frequency band, so as to attain thestability of the circuit.

DISCLOSURE OF THE INVENTION

In order to accomplish the above object, in a radio frequency circuit ora digital circuit, a reflection loss suppression circuit of the presentinvention comprises a transmission line which transforms an outputimpedance or an input impedance in a specified frequency band into ahigh impedance; and a resistance grounding circuit having a frequencyselectivity, which is connected in parallel with the transmission lineas viewed from an output terminal side or an input terminal side. Theresistance grounding circuit includes a resistor having a predeterminedresistance in the vicinity of a load resistance or a signal sourceresistance, and has a low impedance in the specified frequency band. Theresistance grounding circuit is constructed of a circuit which isterminated by a one-terminal pair circuit having a high impedanceoutside the frequency band.

The one-terminal pair circuit is constructed of an end-open stub whichhas a length equal to a quarter wavelength of a fundamental wave at anydesired frequency included in the specified frequency band.

The one-terminal pair circuit is constructed of a circuit in which acapacitance element is terminated by an end-open stub shorter by δ thana half wavelength of a fundamental wave, when δ is let denote a lengthbeing sufficiently small as compared with a wavelength λ of thefundamental wave at any desired frequency included in the frequencyband. At an angular frequency ω₀ corresponding to the frequency, acapacitance C of the capacitance element, a characteristic impedance Z₀and a propagation constant γ of the end-open stub, and the length δ areselected so as to satisfy:Im[tan h{γ(λ/2−δ)}]=−ω₀ ×C×Z ₀.

The one-terminal pair circuit is constructed of a circuit in which aninductor element is terminated by an end-open stub longer by δ than ahalf wavelength of a fundamental wave, when δ is let denote a lengthbeing sufficiently small as compared with a wavelength λ of thefundamental wave at any desired frequency included in the frequencyband. At an angular frequency ω₀ corresponding to the frequency, aninductance L of the inductor element, a characteristic impedance Z₀ anda propagation constant γ of the end-open stub, and the length δ areselected so as to satisfy:Im[tan h{γ(λ/2+δ)}]=Z ₀/(ω₀ ×L).

The transmission line and the end-open stub are formed by employingmicrostrip lines or coplanar lines.

In a radio frequency circuit or a digital circuit, a reflection losssuppression circuit in the present invention comprises a transmissionline which transforms an output impedance or an input impedance in aspecified frequency band into a high impedance, and a resistancegrounding circuit having a frequency selectivity, which is connected inparallel with the transmission line as viewed from an output terminalside or an input terminal side. The resistance grounding circuit isconstructed of a circuit in which a band-pass filter for passing asignal of the frequency band is grounded through a resistor having apredetermined resistance in the vicinity of a load resistance or asignal source resistance.

The band-pass filter is constructed of an interdigital capacitor whichhas a length equal to a quarter wavelength of a fundamental wave at anydesired frequency included in the specified frequency band.

An electric length of the transmission line is set in a range of ±50% ofan absolute value of an output impedance with the transmission lineviewed. A resistance of the resistor constituting the resistancegrounding circuit should preferably be set in a range of 0.5 through 2times the load resistance or the signal source resistance.

A wideband amplifier according to the present invention comprises thereflection loss suppression circuit at, at least, one of an output endand an input end.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram for explaining a distributed amplifier inthe prior art.

FIG. 2 is a circuit diagram for explaining the arrangement of areflection loss suppression circuit according to the first embodiment ofthe present invention.

FIG. 3 is a circuit diagram for explaining the arrangement of areflection loss suppression circuit according to the second embodimentof the present invention.

FIG. 4 is a circuit diagram for explaining the arrangement of areflection loss suppression circuit according to the third embodiment ofthe present invention.

FIG. 5 is a circuit diagram showing the layout of the resistancegrounding circuit of the reflection loss suppression circuit accordingto the third embodiment of the present invention.

FIG. 6 is a model diagram for explaining the effect of the presentinvention.

FIG. 7 is a diagram in which reflection coefficients Γ within afrequency band to have a reflection loss suppressed therein are plottedon a Smith chart.

FIG. 8 is a diagram for explaining the effect of the present invention,and is a graph showing the simulation result of the frequency dependencyof a gain.

FIG. 9 is a diagram for explaining the effect of the present invention,and is a graph showing the simulation result of the frequency dependencyof an output side reflection loss.

FIG. 10 is a diagram for explaining the effect of the present invention,and is a graph showing the simulation result of the frequency dependencyof a stability factor.

FIG. 11 is a diagram for explaining the effect of the present invention,and is a graph showing the simulation result of the frequency dependencyof a stability measure.

FIG. 12 is a circuit diagram for explaining the arrangement of areflection loss suppression circuit according to the fourth embodimentof the present invention.

FIG. 13 is a circuit diagram showing the layout of the resistancegrounding circuit of the reflection loss suppression circuit accordingto the fourth embodiment of the present invention.

FIG. 14 is a model diagram for explaining the effect of the presentinvention.

FIG. 15 is a circuit diagram for explaining the arrangement of areflection loss suppression circuit according to the fifth embodiment ofthe present invention.

FIG. 16 is a circuit diagram for explaining the arrangement of areflection loss suppression circuit according to the sixth embodiment ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, a reflection loss suppression circuit according to the firstembodiment of the present invention will be described with reference toFIG. 2. FIG. 2 is a circuit diagram showing an example in which thereflection loss suppression circuit in this embodiment is connected tothe output end of a distributed amplifier. By the way, in thisembodiment, a portion except the reflection loss suppression circuit 19has the same arrangement as that of the prior-art distributed amplifiershown in FIG. 1, and the same reference numerals are assigned to thesame parts.

The reflection loss suppression circuit 19 in this embodiment isconstructed of a transmission line 15 which is connected in series withthe output end 14 of the distributed amplifier, and a resistancegrounding circuit 18 which has a frequency selectivity and which isconnected in parallel with the transmission line 15 as viewed from theside of the output terminal 8. The electric length θ of the transmissionline 15 is selected within a range of ±50% before and behind a valuewhich maximizes the absolute value of an output impedance Z2 with thetransmission line 15 viewed. The resistance grounding circuit 18 isconstructed of a resistor 16, and a one-terminal pair circuit 17 whichexhibits a low impedance in a frequency band to have a reflection losssuppressed therein and which exhibits a high impedance in any otherfrequency band. The resistance of the resistor 16 is selected within arange of 0.5 through 2 times a load resistance or a signal sourceresistance.

According to the above construction, an output impedance Z1 in thefrequency band to have the reflection loss suppressed therein istransformed into the high impedance Z2 by the transmission line 15, andthe resistance grounding circuit 18 is thereafter connected in parallel.Therefore, the output impedance Z3 of the whole circuit becomessubstantially equal to the value of the resistor 16. Accordingly, thereflection loss can be suppressed by setting the resistance of theresistor 16 in the vicinity of the load resistance, concretely, withinthe range of 0.5 through 2 times the load resistance. Moreover, theabove effect can be realized without affecting circuit characteristicsoutside the frequency band to have the reflection loss suppressedtherein, by sufficiently heightening the frequency selectivity of theresistance grounding circuit 18, namely, the one-terminal pair circuit17.

Next, a reflection loss suppression circuit according to the secondembodiment of the present invention will be described with reference toFIG. 3. FIG. 3 is a circuit diagram showing an example in which thereflection loss suppression circuit in this embodiment is connected tothe output end of a distributed amplifier. Incidentally, the samereference numerals and signs are assigned to the same parts as in thereflection loss suppression circuit shown in FIG. 2.

The reflection loss suppression circuit in this embodiment exhibits alow impedance in a frequency band to have a reflection loss suppressedtherein. A one-terminal pair circuit 17 which exhibits a high impedancein any other frequency band, is constructed of an end-open stub 20 whichhas a length equal to the quarter wavelength of a fundamental wave forany desired frequency included in the first-mentioned frequency band The“wavelength” termed here signifies the effective wavelength of anelectromagnetic wave which propagates through the end-open stub.

Even when the end-open stub 20 having the length equal to the quarterwavelength of the fundamental wave is employed as the one-terminal paircircuit 17 in this manner, the reflection loss can be suppressed as inthe first embodiment stated before. Moreover, the above effect can berealized without affecting circuit characteristics outside the frequencyband to have the reflection loss suppressed therein, by sufficientlyheightening the frequency selectivity of the one-terminal pair circuit17.

Next, a reflection loss suppression circuit according to the thirdembodiment of the present invention will be described with reference toFIG. 4–FIG. 11. FIG. 4 is a circuit diagram showing an example in whichthe reflection loss suppression circuit in this embodiment is connectedto the output end of a distributed amplifier. FIG. 5 is a circuitdiagram of a resistance grounding circuit portion in the reflection losssuppression circuit. FIGS. 6 and 7 are diagrams for explaining theeffects of the present invention. FIGS. 8, 9, 10 and 11 are graphsshowing simulation results concerning a case where the reflection losssuppression circuit in this embodiment is mounted, and a case where itis not mounted. Incidentally, the same reference numerals and signs areassigned to the same parts as in the reflection loss suppression circuitshown in FIG. 2.

The reflection loss suppression circuit in this embodiment exhibits alow impedance in a frequency band to have a reflection loss suppressedtherein. A one-terminal pair circuit 17 which exhibits a high impedancein any other frequency band, is constructed of a capacitor 21 and a(λ/2−δ)-long end-open stub 22 Here, the (λ/2−δ)-long end-open stub 22 isan end-open stub which has a length shorter by δ than the halfwavelength of a fundamental wave for any desired frequency included inthe first-mentioned frequency band. δ denotes a length which issufficiently short as compared with the wavelength λ of the fundamentalwave. The capacitance C of the capacitor 21, the length δ, and thecharacteristic impedance Z₀ and propagation constant γ of the(λ/2−δ)-long end-open stub 22 are selected so as to satisfy Equation (1)below at an angular frequency ω₀ corresponding to the frequency. Here,Im[·] signifies to take the imaginary part of a complex number.

$\begin{matrix}{{{Im}\left\lbrack {\tanh\left\{ {\gamma\left( {\frac{\lambda}{2} - \delta} \right)} \right\}} \right\rbrack} = {{- \omega_{0}}C\; Z_{0}}} & (1)\end{matrix}$

Next, the effects which the reflection loss suppression circuit in thisembodiment has (industrial applicability) will be described withreference to FIGS. 5–11.

FIG. 5 is the diagram showing the layout of a resistance groundingcircuit 18 in the reflection loss suppression circuit 19 shown in FIG.4. An impedance Z_(i2) with the (λ/2−δ)-long end-open stub 22 and thecapacitor 21 viewed is calculated by the following equation (2):

$\begin{matrix}{Z_{12} = {\frac{1}{j\;\omega\; C} + \frac{Z_{0}}{\tanh\left( {\gamma 1}_{-} \right)}}} & (2)\end{matrix}$

Here, 1⁻=λ/2−δ holds. Assuming for the sake of description that the(λ/2−δ)-long end-open stub 22 is of low loss, Equation (2) becomes thefollowing equation (3):

$\begin{matrix}{Z_{12} = {{Z_{0}\frac{{\alpha 1}_{-}}{\sin^{2}{\beta 1}_{-}}} - {j\; Z_{0}\left\{ {\frac{1}{\;{\omega\; C\; Z_{0}}} + {\cot\mspace{11mu}{\beta 1}_{-}}} \right\}}}} & (3)\end{matrix}$

Here, α denotes the attenuation constant of the end-open stub 22, βdenotes the phase constant thereof, and γ=α+jβ holds. Now that 1⁻=λ/2−δholds, β1⁻=β(λ/2−δ) takes a value (π−βδ) which is slightly smaller thanπ, at an angular frequency ω₀. On this occasion, when the capacitance Cis selected so as to satisfy Equation (4) below, two terms constitutingthe imaginary part of Equation (3) balance and cancel each other at theangle frequency ω₀.

$\begin{matrix}{{{- \cot}\;{\beta 1}_{-}} = \frac{1}{\;{\omega_{0}C\; Z_{0}}}} & (4)\end{matrix}$

In general, the loss of a transmission line is small, and the real partof Equation (3) mentioned above is very small, so that Z_(i2) becomessubstantially zero. This situation is shown in model-like fashion inFIG. 6.

On the other hand, when the angular frequency shifts from ω₀, forexample, rises by Δω, the values of the two terms constituting theimaginary part of Equation (3) change in the directions of arrows inFIG. 6. Since the two values having balanced with positive and negativesigns decrease simultaneously, the absolute value of the imaginary partof Equation (3) increases. Here, the increasing rate of the aboveabsolute value of the imaginary part in Equation (3), versus thefrequency, can be made a sufficiently large value by setting δ to besufficiently small as compared with the wavelength λ of a fundamentalwave. Accordingly, the frequency selectivity of the resistance groundingcircuit 18 can be made satisfactorily high.

In the above description, it has been assumed for the sake of brevitythat the (λ/2−δ)-long end-open stub 22 is of low loss. In a case wherethe loss is to be scrupulously considered. In an actual design, theindividual parameters may be determined so as to satisfy Equation (1)instead of Equation (4).

FIG. 7 is such that, in the circuit (circuit in FIG. 4) in which thereflection loss suppression circuit in this embodiment is connected tothe output end of the distributed amplifier, reflection coefficients Γi(i=1, 2, 3) in the frequency band to have the reflection loss suppressedtherein are plotted on a Smith chart. This example handles a case wherethe absolute values of the reflection coefficients exceed one in thefrequency band, that is, a case where a negative resistance hasoccurred. The ensuing description and the effects of the presentinvention hold quite true of a case where the absolute values of thereflection coefficients do not exceed one, that is, a case where thenegative resistance has not occurred.

As shown in FIG. 7, the reflection coefficient Γ1 as to which thereflection loss has increased and led to the occurrence of the negativeresistance in the frequency band is moved near to a point at infinity onthe Smith chart by the transmission line 15. That is, an input impedanceZ1 is transformed into a high impedance Z2. Subsequently, the resistancegrounding circuit 18 having the intense frequency selectivity isconnected in parallel as stated before, whereby the reflectioncoefficient Γ2 is transformed into Γ3 near the center of the Smithchart. That is, the high impedance Z2 is transformed into an impedanceZ3 which is close to a load impedance. Accordingly, the reflection lossin the frequency band is suppressed.

FIGS. 8 and 9 are graphs showing the simulation results of the frequencydependencies of a gain |S21| and an output side reflection loss |S22|,respectively, as to the case (the present invention) where thereflection loss suppression circuit in this embodiment is mounted, andthe case (the prior art shown in FIG. 1) where it is not mounted. It isseen from both the figures that the reduction of the output sidereflection loss |S22| outside the required band is achieved withoutappreciably affecting the gain |S21| characteristic within the requiredband, by mounting the reflection loss suppression circuit in thisembodiment.

FIGS. 10 and 11 are graphs showing the simulation results of thefrequency dependencies of a stability factor and a stability measure,respectively, as to the case (the present invention) where thereflection loss suppression circuit in this embodiment is mounted, andthe case (the prior art shown in FIG. 1) where it is not mounted. Whenthe stability factor is larger than one, and when the stability measureis larger than zero, the circuit has an absolute stability. As seen fromboth the figures, the absolute stability is not attained in the case(the prior art) where the reflection loss suppression circuit is notmounted, whereas the absolute stability is attained by mounting thereflection loss suppression circuit in this embodiment.

Next, a reflection loss suppression circuit according to the fourthembodiment of the present invention will be described with reference toFIG. 12–FIG. 14. FIG. 12 is a circuit diagram showing an example inwhich the reflection loss suppression circuit in this embodiment isconnected to the output end of a distributed amplifier. Besides, FIG. 13is a circuit diagram of a resistance grounding circuit portion in thereflection loss suppression circuit. FIG. 14 is a graph showing thesimulation results concerning a case where the reflection losssuppression circuit in this embodiment is mounted, and a case where itis not mounted. Incidentally, the same reference numerals and signs areassigned to the same parts as in the reflection loss suppression circuitshown in FIG. 2.

The reflection loss suppression circuit in this embodiment exhibits alow impedance in a frequency band to have a reflection loss suppressedtherein. A one-terminal pair circuit 17 which exhibits a high impedancein any other frequency band, is constructed of an inductor 23 and a(λ/2+δ)-long end-open stub 24.

Here, the (λ/2+δ)-long end-open stub 24 is an end-open stub which has alength longer by a than the half wavelength of a fundamental wave forany desired frequency included in the first-mentioned frequency band. δdenotes a length which is sufficiently short as compared with thewavelength λ of the fundamental wave. The inductance L of the inductor23, the length δ, and the characteristic impedance Z₀ and propagationconstant γ of the (λ/2+δ)-long end-open stub 24 are selected so as tosatisfy Equation (5) below at an angular frequency ω₀ corresponding tothe frequency. Here, Im[•] signifies to take the imaginary part of acomplex number.

$\begin{matrix}{{{Im}\left\lbrack {\tanh\left\{ {\gamma\left( {\frac{\lambda}{2} + \delta} \right)} \right\}} \right\rbrack} = \frac{Z_{0}}{\omega_{0}L}} & (5)\end{matrix}$

Next, the effects which the reflection loss suppression circuit in thisembodiment has (industrial applicability) will be described withreference to FIGS. 13 and 14.

FIG. 13 is the diagram showing the layout of a resistance groundingcircuit 18 in the reflection loss suppression circuit 19 shown in FIG.12. An impedance Z_(i2) with the (λ/2+δ)-long end-open stub 24 and theinductor 23 viewed is calculated by Equation (6) below. Here, 1₊=λ/2+δholds.

$\begin{matrix}{Z_{12} = {{j\;\omega\; L} + \frac{Z_{0}}{\tanh\left( {\gamma 1}_{+} \right)}}} & (6)\end{matrix}$

Assuming for the sake of description that the (λ/2+δ)-long end-open stub24 is of low loss, Equation (6) becomes Equation (7) below. Here, αdenotes the attenuation constant of the end-open stub 24, β denotes thephase constant thereof, and γ denotes a value satisfying γ=α+jβ.

$\begin{matrix}{Z_{12} = {{Z_{0}\frac{{\alpha 1}_{+}}{\sin^{2}{\beta 1}_{+}}} + {j\; Z_{0}\left\{ {\frac{\omega\; L}{\mspace{11mu} Z_{0}} - {\cot\mspace{11mu}\beta\; L_{+}}} \right\}}}} & (7)\end{matrix}$

Now that 1₊=λ/2+δ holds, β1₊=β(λ/2+δ) takes a value (π+βδ) which isslightly larger than π, at an angular frequency ω₀. On this occasion,when the inductance L is selected so as to satisfy Equation (8) below,two terms constituting the imaginary part of the above equation (7)balance and cancel each other at the angle frequency ω₀.

$\begin{matrix}{{\cot\mspace{11mu}{\beta 1}_{+}} = \frac{\omega_{0}}{Z_{0}}} & (8)\end{matrix}$

In general, the loss of a transmission line is small, and the real partof Equation (7) is very small, so that Z_(i2) becomes substantiallyzero. This situation is shown in model-like fashion in FIG. 14.

On the other hand, when the angular frequency rises by Δω from ω₀, thevalues of the two terms constituting the imaginary part of Equation (7)change in the directions of arrows in FIG. 14. Since the two valueshaving balanced with positive and negative signs decreasesimultaneously, the absolute value of the imaginary part of Equation (7)increases. Here, the increasing rate of the above absolute value of theimaginary part in Equation (7), versus the frequency, can be made asufficiently large value by setting δ to be sufficiently small ascompared with the wavelength λ of a fundamental wave. Accordingly, thefrequency selectivity of the resistance grounding circuit 18 can be madesatisfactorily high.

In the above description, it has been assumed for the sake of brevitythat the (λ/2+δ)-long end-open stub 24 is of low loss. In a case wherethe loss is to be scrupulously considered in an actual design, theindividual parameters may be determined so as to satisfy Equation (5)instead of Equation (8).

Next, a reflection loss suppression circuit according to the fifthembodiment of the present invention will be described with reference toFIG. 15. FIG. 15 is a circuit diagram showing an example in which thereflection loss suppression circuit in this embodiment is connected tothe output end of a distributed amplifier. Incidentally, the samereference numerals and signs are assigned to the same parts as in thereflection loss suppression circuit shown in FIG. 2.

The reflection loss suppression circuit in this embodiment features thepoint that a resistance grounding circuit 18 having a frequencyselectivity is constructed of a circuit in which a band-pass filter 25for passing the signal of a frequency band to have a reflection losssuppressed therein is grounded through a resistor 16. The reflectionloss can be suppressed even by such a construction. The effects statedbefore can be realized without affecting circuit characteristics outsidethe frequency band to have the reflection loss suppressed therein, bysufficiently heightening the frequency selectivity of the band-passfilter 25.

Next, a reflection loss suppression circuit according to the sixthembodiment in the present invention will be described with reference toFIG. 16. FIG. 16 is a circuit diagram showing an example in which thereflection loss suppression circuit in this embodiment is connected tothe output end of a distributed amplifier. Incidentally, the samereference numerals and signs are assigned to the same parts as in thereflection loss suppression circuit shown in FIG. 15.

The reflection loss suppression circuit in this embodiment features thepoint that the band-pass filter 25 in the fifth embodiment isconstructed of an interdigital capacitor 26. The reflection loss can besuppressed even by such a construction. The effects stated before can berealized without affecting circuit characteristics outside a frequencyband to have the reflection loss suppressed therein, by sufficientlyheightening the frequency selectivity of the interdigital capacitor 26.

Incidentally, the above embodiments have been described by exemplifyingthe distributed amplifier as the circuit from which the reflection lossis to be suppressed. However, the present invention is not restricted tothe embodiments, but it is applicable in various circuits such asamplifiers, oscillators, mixers, frequency multipliers and frequencydividers of all types, and further, multifarious digital circuits as maybe needed.

Besides, in the above embodiments, the HBTs (Hetero-junction BipolarTransistors) are employed as basic elements. The present invention,however, can also be applied to circuits which employ any sorts ofdevices including FETs such as MESFETs (Metal Semiconductor Field EffectTransistors) or HEMTs (High Electron Mobility Transistors), and siliconbipolar transistors.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, an output orinput impedance in a frequency band to have a reflection loss suppressedtherein is transformed into a high impedance by a transmission linehaving a suitable electric length, and a resistance grounding circuithaving a high frequency selectivity is thereafter connected in parallel,whereby the reflection loss in the desired frequency band outside arequired band can be reduced without affecting circuit characteristicsin the required frequency band. Further, the stability of a circuit canbe enhanced owing to the reduction of the reflection loss.

1. A reflection loss suppression circuit, comprising: a transmissionline which transforms an output impedance or an input impedance in aspecified frequency band into a high impedance; and a resistancegrounding circuit having frequency selectivity, which is connected inparallel with said transmission line as viewed from an output terminalor an input terminal of said reflection loss suppression circuit,wherein: said resistance grounding circuit comprises a resistor having apredetermined resistance in the vicinity of a load resistance or asignal source resistance, and having a low impedance in the specifiedfrequency band, a capacitance element, and an end-open stub connected inseries to provide a high impedance outside said frequency band, saidend-open stub is shorter by δ than a half wavelength of a fundamentalwave, where δ is a length sufficiently small as compared with awavelength λ of the fundamental wave at any desired frequency includedin said frequency band; and at an angular frequency ω₀ corresponding tothe desired frequency, a capacitance C of said capacitance element, acharacteristic impedance Z₀ and a propagation constant γ of saidend-open stub, and the length δ are selected so that Im[tanh{γ(λ/2−δ)}]=−ω₀×C×Z₀.
 2. A reflection loss suppression circuit asdefined in claim 1, wherein said transmission line and said end-openstub comprise microstrip lines.
 3. A reflection loss suppression circuitas defined in claim 1, wherein said transmission line and said end-openstub comprise coplanar lines.
 4. A reflection loss suppression circuitfor use with a radio frequency circuit or a digital circuit, saidreflection loss suppression circuit comprising: a transmission lineconnected to an output of the radio frequency circuit or the digitalcircuit, to transform an output impedance or an input impedance of theradio frequency circuit or digital circuit in a specified frequency bandinto a high impedance; a resistor having a predetermined resistance inthe vicinity of a load resistance or a signal source resistance of theradio frequency circuit or digital circuit, and having a low impedancein the specified frequency band; a capacitance element; and an end-openstub, wherein: said resistor, said capacitance element, and saidend-open stub are connected in series to provide a resistance groundingcircuit having frequency selectivity and having a high impedance outsidesaid frequency band, and with said resistance grounding circuitconnected in parallel with said transmission line, said end-open stub isshorter by δ than a half wavelength of a fundamental wave, where δ is alength sufficiently small as compared with a wavelength λ of thefundamental wave at any desired frequency included in said frequencyband; and at an angular frequency ω₀ corresponding to the desiredfrequency, a capacitance C of said capacitance element, a characteristicimpedance Z₀ and a propagation constant γ of said end-open stub, and thelength δ are selected so that Im[tan h{γ(λ/2−δ)}]=−ω₀×C×Z₀.
 5. Areflection loss suppression circuit as defined in claim 4, wherein saidtransmission line and said end-open stub comprise microstrip lines.
 6. Areflection loss suppression circuit as defined in claim 4, wherein saidtransmission line and said end-open stub comprise coplanar lines.